Most recent mumps outbreaks occurred in people aged 18-22 who had received the requisite two doses of mumps vaccine in childhood.

Perri Klass, M D, New York Times, Nov 6, 2017

If you think you've been seeing mumps in the news more often in the past couple of years, you're absolutely right.

"Mumps outbreaks are on the rise," said Dr. Janell Routh, a pediatrician who is a medical officer on the mumps team at the Centers for Disease Control and Prevention. More than 6,000 cases of mumps were reported in the United States last year, the highest number in 10 years. Around 2010, total annual cases were down in the hundreds.

Most of the recent cases occurred in outbreaks, including a large one in Arkansas, rather than as a sporadic here-a-case, there-a-case disease. And most of the outbreaks were among people 18 to 22 years old, most of whom had had the requisite two doses of mumps vaccine in childhood. "We are seeing it in a young and highly vaccinated population," Dr. Routh said.

In my world, you can date people by their childhood diseases. I'm too young to have had measles, but old enough to have had mumps and chickenpox. Chickenpox I remember as particularly itchy and unpleasant; mumps I remember for the swollen chipmunk cheeks and, as with tonsillitis, a certain amount of ice cream to make painful swallowing easier.

Mumps is a virus that causes swelling of the parotid glands, the salivary glands under the ears, along with some more general symptoms like fever and fatigue. The mumps vaccine was licensed in 1967, based on the Jeryl Lynn strain of mumps, and developed by Dr. Maurice Hilleman, who cultured the virus from his sick daughter.

Mumps is transmitted by droplets of saliva or mucus. It can be spread by coughing and sneezing, but also by sharing cups and the close contact of living and eating and exercising together. Many of the recent outbreaks occurred in college dorms or among athletic teams, as happened with the Syracuse University men's and women's lacrosse teams last month. And "we are seeing it in other close-knit communities that tend to live closely together with strong social or cultural interactions," Dr. Routh said, including religious groups.

Mumps vaccine is now combined with measles and rubella vaccine in the M.M.R., given at ages 1 and 4. But the immunity wanes over time in some people, and with close contact, there can be sufficient exposure to sufficient quantities of virus to overcome the vaccines' protection.

Dr. Patricia Quinlisk, the medical director and state epidemiologist for the Iowa Department of Public Health, dealt with an outbreak at the University of Iowa and surrounding area in 2015 to 2016 of more than 450 cases of mumps. The students involved had all had their childhood M.M.R. shots, she said, as required by the university, and the decision was made to hold a series of clinics offering a third dose of vaccine.

In an article in September in the New England Journal of Medicine, Dr. Quinlisk and her colleagues traced the importance of waning immunity, and the effectiveness of that third M.M.R. shot in the Iowa outbreak.

"Should this situation occur again, we would give a third dose," Dr. Quinlisk said. "In our outbreak it did substantially decrease the risk of other students getting mumps, and was instrumental in stopping the outbreak."

Other strategies to contain the outbreak on campus included education about cough hygiene (for instance, cough into your arm or a tissue), and working with student health services to make it possible for infectious students to isolate themselves, arranging for them to miss classes and retake tests so there was no pressure to break isolation, setting up food delivery services, and providing masks for those who had to use communal bathrooms.

Even if the immunity has dwindled, "we know that two doses of M.M.R. decreases your risk of serious complications," Dr. Routh said. An important rationale for vaccinating against mumps was always to protect against orchitis, an inflammation of the testicles which used to be relatively common in post-pubertal males who came down with mumps, affecting 20 to 30 percent of them, and sometimes leaving them with loss of testicular tissue, decreased sperm counts and concerns about future fertility. The rate was much lower in vaccinated young adults.

Mumps virus can also more rarely cause inflammation of the ovaries, and even more rarely, though more dangerously, deafness and inflammation of the brain; again, these complications appear to be even rarer in those who have been vaccinated, but they may still occur. And even with 6,000 cases, it can be hard to estimate the frequency of a complication like lasting deafness, which used to occur in about one in every 20,000 cases.

Of course, waning immunity can be a problem after college as well. Dr. Alan Lucerna, the program director for combined emergency and internal medicine at Rowan School of Osteopathic Medicine in New Jersey, reported in February 2017 on a case of mumps in a fully immunized adult, in an article titled "Still a pain in the neck after all this time."

"A good number of patients who come to the E.R. and are subsequently diagnosed with mumps are vaccinated and come in thinking it was just a lymph node swelling," Dr. Lucerna said. Adults who develop swelling at the angle of the jaw together with flulike symptoms should think about mumps, he said. "We probably underdiagnose it."

Should parents worry about sending their children off to college with their two childhood doses of mumps vaccine? "No, we do know that two doses of vaccine is protective," Dr. Routh said. The current recommendation is that a third dose, while safe, is only warranted for people felt to be at high risk by public health workers, like the Iowa students in the setting of an outbreak. The two childhood doses of M.M.R. do protect most people against getting mumps, and they also, of course protect against measles, a much more serious disease, not to mention rubella.

"We've heard some people say this proves the vaccine's not working, but the vaccine probably prevented a lot more students from getting ill," Dr. Quinlisk said. Immunity does not wane in everyone, so many people who are exposed don't get sick. "We certainly commend the University of Iowa for having a mandatory vaccine policy which kept it from being worse and prevented complications."

Parents might want to keep in mind the informal survey done at the Iowa vaccine clinics where students waiting to get their third M.M.R.s were asked why they had come: Were they motivated by the posters on campus bulletin boards or by fear of complications?

"The No. 1 answer by far was: My mother called me and told me to," Dr. Quinlisk said. "So the emails that went out to parents had a significant impact, and we have told other college campuses to make sure they don't forget to keep the parents in the loop."